VK4WDM wrote: copper earthing strap 7cm wide 1cm thick
Obviously there has to be compromise with amateur stations because of the costs and complexity involved. My mast and anything else that needs earthing is done with 12mm diameter multi-strand power cord. At VK4KG we use off cuts of 25mm dia heliax with plastic removed the inner and outer connected together.
73
Wayne VK4WDM
I suggest you strike up friendship with local SLM/FIM/First responder and go check National sites out to see how minimum contractual lighting protection is done.
All earthing goes to common point, yes we use big green cables like you say but the primary earthing is done by thick copper straps, both on towers and along cable trays.
The earth straps are insulated until point of grounding at central station earth.
We do triennial ground inspections to make sure they are sound and fit for use.
Get it wrong and ???
Cows sheltered near fence in storm that got hit.
Lightning experts from the Electrical & Computer Engineering department are studying ways to reduce the cost of lightning damage by investigating how to repair and protect power lines that are hit by lightning.
Lightning damage to power lines in the U.S. costs almost $1 billion annually, and 30 percent of all power outages are lightning related, according to studies by the Electric Power Research Institute (EPRI). The cost to industry of power failures led EPRI in 1993 to create the predecessor to what a year later became UF’s International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida.
“Industry wants power that stays steady at 60 cycles per second, with no impulses or failures caused by lightning,” says Martin Uman, UF Electrical & Computer Engineering (ECE) professor. “Power should be pure and uninterrupted,” says Uman, who, with ECE Professor Vladimir Rakov, is co-director of the ICLRT.
The ICLRT’s triggered lightning experiments involve underground and overhead power distribution lines built on the site. Presently, there are two different types of overhead lines that are identical to those used by power companies such as Florida Power & Light, which funds the present experiments.
Lightning is triggered by wire-trailing rockets fired toward overhead storm clouds. Lightning vaporizes the wires and follows their traces down in a manner that can be observed and studied. The researchers have investigated what happens when lightning hits power lines, how they fail, how to fix them, how to protect them, and equally important, the physics of that lightning.
A power line collects natural lightning that would otherwise strike in an alley about 100 feet wide on either side of the line. To protect against the effects of lightning strikes, power lines are often equipped with arresters and overhead ground wires. An arrester behaves like an open circuit when the line is in normal operation. When the line is hit by lightning, the arrester acts more like a short circuit. Like throwing a switch, it diverts lightning current to the ground, holds the voltage at safe value, and thereby keeps the line from harm.
The tests at the ICLRT revealed that up to half of all natural lightning strikes will cause the nearest arresters to fail, according to Uman.
The studies revealed that underground power lines are also vulnerable, contrary to previous beliefs, Rakov says. Ground lightning strikes affect underground distribution lines almost as often as they affect overhead lines. Further, the ground rods intended to dissipate the current actually act as interceptors.
A percentage of the current does enter the system through the ground rods and can cause damage in the system.
As a further test, the ICLRT conducted an experiment on ground rods of the kind commonly used to protect homes from lightning.
The observation that ground rods appeared to transmit the bulk of the current from lightning into the system rather than to “ground” was unexpected.
The researchers built a small structure with a typical lightning rod system at the ICLRT. Lightning rods placed on a home’s roof are supposed to route the current from a strike into the soil via wires connected to buried vertical ground rods. The established international standard for these systems allows no more than 50 percent of a lightning strike’s current to enter a home’s electrical system.
The experiment demonstrated that more than 80 percent of current from a lightning strike flowed into an electrical system when the ground rods were in sandy soil of the kind found in the southeastern states. Sandy soil tends to remain dry beneath the surface and therefore does not conduct electricity well.
Rakov says more research needs to be done to determine how lightning current is distributed through a system. The ICLRT studies have shown that in 50 percent of lightning strikes, the strong initial pulse of the current is followed by a tail of continuing current of variable duration. It is not now known how the continuing currents divide or if they flow through arresters. This is important because arresters are designed for and tested against strong pulses only, Rakov says.
In the meantime, the studies indicate that homeowners should probably use surge protectors at the electric meter and in the home.
Wire ring grounding systems are also desirable. These systems involve lightning rods connected to a buried wire loop that circles the house. Because the loops have more surface area than ground rods, they can better dissipate current into the ground.
As for power lines, the solution lies with the power companies, Uman says, given the available results of the experiments. It is theoretically possible to make power lines lightning proof if overhead ground wires are combined with line arresters of the proper power and energy rating. And although there are several types of new lightning elimination or dissipation devices now sold that are advertised as able to protect against lightning by diverting it away from the power lines, they have not been proven to work in that situation, he says.
Writer: Martha Dobson